Has anyone used one of these as a walk around lens and does the weight get to be a problem after a while. Also, are they hard to operate or are they just like standard lens? I have a Tamron 18-270 lens but I am to the point I want professional quality and a fixed f-stop value throughout. Also I see there are series II, and also a 4L. I have read the series II is alot better but I haven't been able to figure out what the 4L is. Thanks